from gi.repository import Gtk
from gi.repository import Gdk

def _destroy_cb(widget, data=None):
    Gtk.main_quit()

window = Gtk.Window()
window.resize(500, 300)
window.connect("destroy", _destroy_cb)

screen = Gdk.Screen.get_default()

css_provider = Gtk.CssProvider()
css_provider.load_from_path('style.css')

context = Gtk.StyleContext()
context.add_provider_for_screen(screen, css_provider,
                                Gtk.STYLE_PROVIDER_PRIORITY_USER)

box = Gtk.Box(Gtk.Orientation.VERTICAL, 0)
window.add(box)

label = Gtk.Label()
label.set_text("Hello World")
box.pack_start(label, False, False, 0)

window.show_all()
Gtk.main()
